Communication systems that operate at radio frequencies (RF) require small and low cost bandpass filters and oscillators. These bandpass filters and oscillators are required to generate, select transmit or receive signals within a certain band width (BW) at a specified frequency and to reject signals at frequencies outside the band width. Some examples include global positioning systems (GPS), mobile telecommunication systems: Global Systems for Mobile Communications (GSM), personal communication service (PCS), the Universal Mobile Telecommunications System (UMTS), Long Term Evolution Technology (LTE), data transfer units: Bluetooth, Wireless Local Area Network (WLAN), and satellite broadcasting and future traffic control communications. The bandpass filters are also used in other high frequency systems for high rate data transmission/acquisition in air and space vehicles.
There are few types of bandpass filters and oscillators for RF signal filtering that are fabricated using different technologies: (a) ceramic filters oscillators based on dielectric resonators; (b) filters or oscillators using surface acoustic wave resonators (SAW); and (c) filters or oscillators using thin film bulk acoustic wave resonators (FBAR). For simplicity reasons, the following description will concentrate on RF filters as the main principles of oscillators may be derived easily from this for those skilled in the art. In mobile communication systems such as handsets, the power capability required for the RF filters is about 5 W or less which is not high, but the size and cost requirements are quite critical. Because of this, the RF filters in handsets are usually manufactured on wafers by microelectronic fabrication processes and they either take a SAW form or a FBAR form based on piezoelectric properties of materials. The main properties of piezoelectric materials for filters are propagation velocity of acoustic waves which determines the resonant frequency along with electrode pitch and the coupling coefficients which affect the band width. FIG. 1A shows a schematic diagram of a prior art surface acoustic wave filter (SAW 100a) on a piezoelectric substrate (110S). It comprises an input inter digital transducer IDT1 (120S) with a center-to-center distance between adjacent electrodes controlled to a “pitch” and an output inter digital transducer IDT2 (150S) with a center-to-center distance between adjacent electrodes again controlled to the “pitch”. The IDT1 (120S) is connected to an electrical signal source (130S) to excite acoustic waves (140S) with a velocity v and at a frequency fo=v/(2×pitch). The IDT2 (150S) is to receive the acoustic waves (140S) and covert them into an output electrical signal (160S). Electrical signals in the signal source (130S) at frequencies other than fo cannot excite resonant acoustic waves with sufficient level to reach the output inter digital transducer (150S) to generate an output in the output terminals. Therefore, once a SAW filter has been fabricated, the central frequency fo of transmission signals and bandwidth BW are fixed by the geometry and materials used. Only the electrical signals at fo and within the bandwidth BW are allowed to reach the output inter digital transducer (120S) from the input inter digital transducer (150S).
Velocities of acoustic waves in piezoelectric materials are important for designing acoustic filters. Values for several piezoelectric substrates are given here: ˜4,000 m/s for LiNbO3, ˜6,300 m/s for ZnO, ˜10,400 m/s for AlN and ˜7,900 m/s for GaN. To obtain a filter on LiNbO3 with a central frequency fo=2 GHz, the wavelength of the acoustic wave is λ=(4000 msec)/(2×109/sec)=2×10−4 cm. The value of electrode pitch in FIG. 1A is then equal to 1 μm. Assume that the width of electrodes and space between adjacent electrodes are equal, then the electrode width is 0.5 μm. To fabricate IDTs at higher frequencies, more advanced lithography tools and more severe processing control will be needed. As the width of electrodes is reduced to below 0.5 μm, unwanted series resistance associated with the electrodes will increase to degrade the performance of the SAW filters. Therefore, SAW filters as shown in FIG. 1A are often limited to applications at frequencies near or below 1 GHz. To maintain the performance for operation at high frequencies and to reduce the manufacturing cost, another filter structures have been developed: the film bulk acoustic resonator (FBAR). The acoustic filters based on the FBAR are capable of operation at frequencies from a few hundred MHz to 40 GHz. It is noted that SAW filters have a footprint which is about twice of that for FBAR filters with similar electrical performance at frequencies of about of 1 GHz.
FIG. 1B shows a schematic cross-sectional diagram of a prior art film bulk acoustic resonator FBAR (100b) on a substrate (110) having a substrate thickness (110t). The FBAR (100b) comprises an air cavity (115) having an air cavity depth (115t), a bottom electrode (140) with a bottom electrode thickness (140t), a piezoelectric layer (150) with a piezoelectric layer thickness (150t), a top electrode (160) with a top electrode thickness (160t). The purpose of the air cavity (115) is to prevent the acoustic waves (185) from getting into the substrate (110) and to confine them in the piezoelectric layer (150). The resonant frequency is mainly determined by the piezoelectric layer thickness (150t). Through isolation of the air cavity (115), the acoustic energy is confined in the piezoelectric layer (150) with minimum loss into the substrate. However, due to the presence of the air cavity (115), the dissipation of heat generated in the piezoelectric layer (150) and the metal electrodes (140, 160) to the substrate (110) of the FBAR is limited, as there is no other major heat conduction paths. Therefore, the operating power of systems involving the FBAR with the air cavity (115) can not be too high in order to prevent the instability of the circuits due to excessive heating of the piezoelectric layer (150). To improve dissipation of heat from the piezoelectric layer, another structure of FBAR has been adopted and used in RF filters.
FIG. 1C shows a schematic cross-sectional diagram of another prior art FBAR which is called a solidly mounted bulk acoustic resonator (SMBAR or SMR) (100c) as it is solidly mounted on a substrate. The SMBAR is deposited on the substrate (110) with a substrate thickness (110t). It comprises a thin film reflector stack (120) having alternative low impedance layers (120-L1, 120-L2, 120-L3) with a low impedance layer thickness (120-Lt) and high impedance layers (120-H1, 120-H2) with a high impedance layer thickness (120-Ht); a total thin film reflector stack thickness (120Tt); a bottom electrode (140) with a bottom electrode thickness (140t); a piezoelectric layer (150) with a piezoelectric layer thickness (150t); a top electrode (160) with a top electrode thickness (160t). The resonant frequency fo is determined by acoustic waves (185) of a velocity v and the piezoelectric layer thickness (150t). To confine the acoustic wave energy in the piezoelectric layer and to minimize the loss to the substrate (110), low impedance layer thickness (120-Lt) and high impedance layer thickness (120-Ht) are selected to be λ/4, here λ is the acoustic wavelength in the respective layers. Since there is no air gap between the piezoelectric layer (150) and the substrate (110), the dissipation rate of heat generated in the piezoelectric layer (150) and the electrode layers (140, 160) to the substrate (110) is greater than that of the FBAR (100b) with an air gap (115) as shown in FIG. 1B. Once a filter involving FBARs has been fabricated, the central frequency fo of transmission and the bandwidth BW are determined by the geometry and materials used.
The frequencies and bandwidths of RF signals for communications have been defined and assigned by regions or countries. For mobile communications, there are about 40 bands. More bands are expected for the next generation long term extension technology. Table 1 gives several selected bands for mobile communications used in different regions or countries. In each band there is a transmit band (Tx Band) at foTR with a transmit band width (BWTR). There is also an associated receive band (Rx Band) at foRE with a receive band width (BWRE). The separation between the transmit band and receive band is given by the difference between the transmit band central frequency foRE and the receive band central frequency foTR: foRE-foTR.
TABLE 1Band frequencies and bandwidth for some of the Bands assigned to mobile handsets and base stations.BandfoTR (MHz)BWTR (MHz)foRE (MHz)BWRE (MHz)foRE − foTR (MHz)Region11920-1980602110-217060190Asia, EMEA, Japan21850-1910601930-19906080N. America, Latin Am.31710-1785751805-18807595Asia, EMEA41710-1755452110-215545400N. America, Latin Am.5824-84925869-8942545N. America, Latin Am.72500-2570702620-269070120Asia, EMEA8880-91535925-9603545EMEA, Latin Am.12699-71617729-7461730N. America
FIG. 1D demonstrates the narrow frequency bands for transmit and receive in wireless communication system, showing a central frequency for transit foTR, a transmit band width BWTR, a central frequency for receive foRE, and a receive band width BWRE. The difference of central frequencies foTR and foRE is selected to be close to BWTR+BWRE to increase the communications capacity. Due to the finite BWTR and BWRE, the separation between the BWTR and BWRE is often small and sometimes an overlap between the two will show. In practical communication systems, the narrow frequency bands capability is implemented using filters with characteristics schematically shown by Curve 1(d) in FIG. 1D for receive band. (Similar characteristic curve for the transmit band can be obtained.) From Curve 1(d), an unwanted attenuation is observed which constitutes an Insertion Loss. This Insertion Loss should be made as small as possible. In Curve 1(d), an unwanted small transmission in the frequency ranges outside the receiving frequency bands is also observed which defines an Isolation. The magnitude of this Isolation should be made as large as possible.
The FBAR (FIG. 1C) resembles a parallel plate capacitor as shown in a simplified diagram in FIG. 1E with a piezoelectric layer (150) between two metal electrodes (140, 160). As stated before, the response of this FBAR is determined by the material properties, especially the properties of the piezoelectric layer; the two electrodes; physical dimensions (the area and thickness) of the piezoelectric layer and the two electrodes.
The variation of the lossless input electrical impedance versus frequency of the FBAR in FIG. 1E is given in FIG. 1F. There are two resonant frequencies close to each other: fa where the impedance approaches infinity and fr where the impedance approaches zero. Between these two frequencies, the FBAR behaves inductively. Outside the band between fr and fa it behaves capacitively with capacitance Co. The resonant frequencies fa and fx of FBAR will determine the bandwidth when a plurality of the FBARs are connected to form a microwave acoustic filter.
There are several wireless standards used in different countries and regions. The main ones are briefly described below.
Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunication Standards Institute to provide protocols for 2G digital cellular networks for mobile phones and first deployed in 1992 in Finland. Recently, GSM has become a global standard for mobile communications operating in many countries and regions.
Personal Communication Service (PCS) describes a set of 3G wireless communications capabilities which allows certain terminal mobility, personal mobility and service management. In Canada, the United States and Mexico, PCS are provided in 1.9 GHz band (1.850-1.990 GHz) to expand the capacity originally provided by the 850 MHz band (800-894 MHz). These bands are particular to the North America although other frequency bands are also used.
The Universal Mobile Telecommunications System or UMTS is a 3G mobile cellular system for networks based on the GSM standard. UMTS uses wideband code division multiple access (W-CDMA) radio access technology to offer greater spectral efficiency and bandwidth to mobile network operators.
Long-Term Evolution (LTE) is a 4G standard for wireless communication with high-speed data for mobile phones and data terminals. It is an upgrade based on the GSM and UMTS network technologies. Different LTE frequencies and bands from about 1 GHz to 3 GHz are used in different countries and regions. There are unlicensed bands in the range from 3 GHz to 6 GHz which maybe used in the near future for mobile communications to increase capacity. Therefore, mobile phones must be equipped with multiple bands modules in order to be used in different countries and regions.
Currently, there are about 40 bands or frequency ranges used for wireless communications in different countries and regions. In the near future, more bands in the frequency range from 3 GHz to 6 GHz are expected due to the need in capacity. Due to the large number of bands used in the mobile handsets in different regions and countries, and even in same country, a practical handset needs to have an RF front end covering several frequency bands. A true world phone will need to have about 40 bands, each with a transmit band and receive band. As each RF filter has only one central frequency of resonant and one bandwidth which are fixed, therefore, such a true world phone will need to have 80 filters for the front end. Due to the resource limitations, some designers design mobile phone handsets to cover 5 to 10 bands for selected regions or countries. Even with this reduced number of bands, the number of RF filters currently required is still large: from 10 to 20 units. Therefore, there are strong needs to reduce the dimensions and cost of the RF filters and to reduce the number of filters for the same number of operation bands by having tunable RF filters, each to cover at least two frequency bands. If this is successful, the number of filters can be reduced in the mobile handsets and many other microwave and wireless systems. Thus, it would be ideal to develop an RF filter which can cover as many bands or frequency ranges as possible so that the size and power consumption of RF front ends in a mobile phone handset and microwave systems can be reduced.